Motor vehicles driven by a combustion engine generally include a manual or automatic gearbox which is interposed in the chain of transmission of the drive torque to the drive wheels of the vehicle. To make it possible to change the gear of the vehicle, it is generally necessary to decouple the engine from the gearbox, in particular by means of a clutch device. In what is referred to as the “normal” mode of operation of the vehicle, the clutch device is as often as possible in the engaged position. The clutch device is generally temporarily disengaged only during gear changes or indeed when the vehicle is stationary.
It is well known that motor vehicles consume less fuel when they are “freewheeling”, i.e. when they are using only their kinetic energy and possibly their weight to travel as far as possible without the engine exerting a braking torque on the wheels of the vehicle, also referred to as “engine braking”.
A method for controlling the vehicle switching from “normal” mode to “freewheel” mode is already known. In this prior art control method, as soon as the conditions are fulfilled that enable the vehicle to continue to travel without the aid of the engine, the clutch device is commanded into the disengaged position and the engine speed is reduced to idle or even zero speed. Such a method is satisfactory in that it saves fuel.
Nevertheless, the sudden opening of the clutch device causes a sudden sensation of deceleration for the passengers of the vehicle.
Furthermore, if the driver of the vehicle decides to return to “normal” mode when the vehicle has just entered “freewheel” mode, the clutch device suddenly returns to the engaged position while the engine is running at idle or even zero speed. This causes a jolt for the passengers in the vehicle.
Such a jolt can be avoided provided that it is waited for the engine to regain a speed corresponding to the speed of rotation of the wheels of the vehicle. Such a delay may be detrimental in certain situations, especially when the switch to “freewheel” mode has been carried out automatically, against the wishes of the driver.